The present invention relates to a temperature detecting device.
An object of the present invention is to provide a highly accurate and inexpensive element for detecting a predetermined temperature and which can be easily fabricated on a semiconductor substrate.
The conventional temperature detecting/controlling system is arranged, for example, as shown in FIG. 1.
The principle of operation of the FIG. 1 system is that the change of the resistance value of a thermistor 1 due to a change of temperature is detected by a resistance value detecting circuit 2, and a driving transistor 4 is turned on or off in accordance with a signal from a control output terminal 4a to control the heating operation of a heater 5. The disadvantages of such a system are as follows:
1. The reliability is relatively low due to deterioration of the thermistor.
2. It is necessary to adjust an adjusting resistor 3 for every thermistor element in such a way that the detecting operation is done at a predetermined temperature because of the large dispersion characteristic of thermistors.
3. A large number of separate parts and a large space are required, the cost of manufacture is high and the reliability is not good.
Therefore, there presently is the need for a temperature detecting device which can be operated in a stable and accurate manner, and which can be easily fabricated on a common substrate at low cost.
The present invention provides a low cost and high reliability device for detecting temperature in which the above mentioned disadvantages are effectively eliminated.
One object of the present invention is to utilize the temperature dependency of forward current of a diode fabricated on a semiconductor as a temperature sensor and to fabricate the temperature detecting device as a monolithic structure.
Another object of the present invention is to provide a voltage measuring circuit capable of detecting the voltage change produced across a diode temperature sensor and which is fabricated on a single substrate to reduce both cost and space.
A further object of the present invention is to provide a circuit for compensating for the dispersion of voltage of a constant voltage measuring circuit and which operates at high accuracy.
The present invention will be described in detail in conjunction with the drawings.
Before describing the present invention, in order to explain one example of a temperature sensor using a diode, an equivalent diode using a silicon NPN junction transistor will first be described. FIG. 2 illustrates a sectional view thereof and FIG. 3 illustrates an equivalent circuit thereof. A base contact P.sup.+ region 7 and an emitter N.sup.+ region 8 are formed in a P well 10 on an N type silicon substrate 6. When the base contact 7 is connected to the collector 6 and a voltage V is applied across a base terminal A and an emitter terminal B, the current I flowing between the emitter 8 and the collector 6 will be expressed as follows in accordance with the standard theory of transistor operation: ##EQU1## in which S: emitter - base junction area;
q: unit charge; PA1 D: diffusion constant of minority carriers; PA1 W: depth of the base (P - well); PA1 X.sub.j : depth of the emitter (N.sup.+ region): PA1 n.sub.i : intrinsic carrier density; PA1 N.sub.A : impurity density of the base (P - well); PA1 k: Boltzmann's constant; PA1 T: temperature;
In addition, n.sub.i and D are as follows: ##EQU2##
A temperature coefficient dV/dT of the voltage across the diode is as follows: ##EQU3##
The equation (4) shows that the temperature coefficient of a diode temperature sensor, illustrated as an equivalent circuit in FIG. 3, is a function of the base concentration N.sub.A and the emitter current density I/S. Therefore, it is possible to quantitatively discuss the dependency of base concentration of temperature coefficient of a silicon planar junction diode temperature sensor shown as an example in FIG. 2. For example, when the base impurity density is 5.times.10.sup.10 atom/cm.sup.3, a constant current I is about 0.1 .mu.A and the emitter-base junction area is 100 .mu.m.times.100 .mu.m, the temperature coefficient will be about 2.8 mV/.degree.C. The dispersion for temperature is small when it is used as a sensor, and it will be within .+-.0.2.degree. C. Although the temperature coefficient is one-fifth that of a thermistor, it has an advantage that it can be fabricated on the same chip in a similar way together with another circuit.